Preparation of photographic emulsions



Patented Dec. 18, 1951 PREPARATION OF PHOTOGRAPHIC EMULSIONS Orion Edwin Schupp, Jr., Wilmington, Del., as-

signor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application May 17, 1946, Serial No. 670,521

6 Claims.

The reasons for its acceptance in spite of its dis advantages are mainly two fold: (l) gelatin has a sharp reversible setting point, and (2) it 0on tains natural photographic sensitizers. The first property makes the preparation of emulsions very much easier since it permits chilling the emulsion to form solid gels which are highly swollen in water but not dissolved so that washing the emulsion is very easy. The sharp gelling point also facilitates coating of the emulsion on the paper or film base. The presen'e of natural sensitizers, of course, permits the on of highly sensitive photographic aterials from gelatin/silver halide systems. From time to time various other colloids have been proposed as substitutes for gelatin. However, none of these has i;

found commercial acceptance since they lack the two above-rnentioned good characteristics of gelatin. One class of materials consic early was the hydrolyzed polyvinyl esters. However, these materials were found to present many difficulties. For example, they were too water soluble to behandled satisfactorily and it was found impossible to control their photographic properties. To avoid these defects it was proposed to treat them with a gelling agent. However, agents which cause satisfactory gelling sad to other defects. For example, phenolic materials tend to give colored emulsions and react with the oxidation products of some developing agents. For these reasons elements based on hydrolyzed polyvinyl esters have not become suocessful commercially' An object of this invention is to provide improved photographic emulsions emulsion layore from polyvinylalcohol. Another object is to provide such emulsion layers which do not deteriorate or wash ofl during photographic processing. Another object is to provide such layers which are of good strength and clarity. A further object is to provide a simple and economical i6 2 process of preparing polyvinyl alcohol-silver halide emulsions and emulsion layers with commercially practical photographic properties. Still other objects will be apparent from the follow ing detailed description of the invention.

It has been found that excellent light-sensh tive photographic emulsions or dispersions can be prepared by dispersing light-sensitive silver halides in an aqueous solution of a selected polyvinyl alcohol, ripening the emulsion, agglutinating the silver halide and polyvinyl alcohol by the addition of an agglutinating or coagulating agent and washing the agglutinate free from the undesirable excess of water-soluble salts. The washed agglutinate is then peptized by heating with stirring with or without the addition of further aqueous material until a smooth, uni.- form dispersion is obtained. The resulting dis.- persions are digested and then coated onto suit.- able supports to form light-sensitive layers. The preparation and coating of the emulsion is car.- ried out in the absence of actinic light.

The selection of the polyvinyl alcohol is. a matter of critical importance. It has been found that it must be free from ester groups. That is, it must be prepared by the complete hydrolysis of a polyvinyl ester and thus has the essential structure (CH2CHOH--)n where n is a large number. The value of n is very important since it has been found that only the higher molecular weight polyvinyl alcohols are useful in this invention. Although the exact determination of molecular weights on high polymers of this type is a matter of considerable difficulty (see, for example, Natural and Synthetic High Polymers, Meyer [Interscience Publishers, New York, 1942], p. 12 et seq.) it has been determined by the osmotic pressure method that the minimum molecular weight of a polyvinyl alcohol suitable for the practice of this invention is approximately 100,000. This corresponds to a degree of polymerization where n is about 2200, i. e., a polymer structure of (CH2CHOH) 2200. The upper limit of utility is not definitely known since it has been impossible to obtain polymers ran-.- dered unusable because of high molecular weight. Nevertheless, it has been determined that poly-.- mers with molecular weights of 160,000 can be used so that the upper limit of utility lies somewhere above this molecular weight value. Since molecular weights are arrived at only by a laborious procedure, a simple viscosity. test has been devised to distinguish useful polyvinyl alcohols. This test is carried out by dissolving ten parts of polyvinyl alcohol in eighty parts of pare the polyvinyl alcohol.

water and ten parts of ethanol with heating at 80 C. for a period of 60 minutes with constant agitation. The solution is then allowed to cool to 25 C. and is maintained at this temperature for a period of several hours. The viscosity of this 10% solution is then measured by the Gardner-Holdt method (see Gardner, Physical and Chemical Examination of Paints, varnishes, Lacquers and Colors, Inst. Paint and Varnish Research, Washington, D. 0., 5th ed., p. 247 (1930). In this way it has been determined that a polyvinyl alcohol with a viscosity of 12 poises is useful whereas a polyvinyl alcohol with a viscosity of poises is unsatisfactory. This sets the lower viscosity limit ith some certainty. The upper limit is not as critical and it has been determined that a polyvinyl alcohol having a viscosity of 41 poises is very satisfactory. A practical viscosity range is from 12 to 50 poises and a preferred range 15 to 50 poises at 25 C.

Another factor to be considered is the manner of hydrolysis of the polyvinyl ester used to pre- It has been discovered that polymers prepared by an alkaline hydrolysis technique are more suitable in the low viscosity ranges, e. g., 12 to 15 poises, than polyvinyl alcohols of the same viscosity prepared by an acidic hydrolytic method. At higher viscosity'levels the difference disappears and acid hydrolyzed polyvinyl alcohols are quite suitable because of their greater ease of solution.

The methods of preparing photographic emulsions from the selected polyvinyl alcohols are quite different from those customarily employed which utilize gelatin. For instance, the polyvinyl alcohols do not gel and it is therefore impossible to chill and noodle the emulsion to obtain suitable washing control as is done with gelatin. The procedures employed in this invention are designed to obtain the emulsion in a form suitable for controlled washing. This is highly important since the speed, contrast, and keeping qualities of polyvinyl alcohol/silver halide emulsions depend directly upon control of the washing step. The method utilized consists essentially of agglutinating the polyvinyl alcohol and dispersed silver halide, converting the agglutinate to a form suitable for controlled washing, followed by a redispersion or peptization of the agglutinate. It is important that the silver halide particles in the agglutinate be not flocculated so that peptization will reproduce the original colloidal condition. The emulsion is then given a suitable digestion and coated on a support.

The photographic dispersions may advantageously be prepared by dissolving polyvinyl alcohol of the type above described as a 3 to 10% solution in pure water with heating and stirring at 70-90 C. for a period of 15 to 120 minutes. Silver halide or a mixture of silver halides is then precipitated in the solution in the absence of actinic radiations. The precipitation may be accomplished by adding to the aqueous solution of the polyvinyl alcohol with stirring an aqueous solution of a water-soluble silver salt, e. g., silver nitrate, and an aqueous solution containing one or more water-soluble halides, e. g., ammonium bromide and potassium iodide. A wetting agent, e. g., sodium dodecyl sulfate, may be used to facilitate the dispersion. Stirring is continued after the salts have been admixed for a period of 15 to 60 minutes until a uniform dispersion is obtained. The emulsion is then ripened. An agglutinating agent for the polyvinyl alcohol and the silver halide is then admixed with the dispersion. This admixture may be effected by adding the agent to the dispersion or by adding the dispersion to a bath comprising the agent, with agitation. Agglutination results in the separation of polyvinyl alcohol containing silver halide particles. At the same time, there is a removal of liquid, so that the concentrations of polyvinyl alcohol and silver halide in the agglutinate are greater than they were in the unagglutinated dispersion. After agglutination is complete, the extraneous liquid is removed and the agglutinate thoroughly washed, e. g., by successive treatments in separate baths of Water or by treatment in a stream of water until the photographic properties attain the desired level. The agglutinate may be pressed to aid in the removal of the washing liquid and soluble salts. The function of the washing step is to remove the major portion of the soluble salts or excess soluble salts present in the emulsion and thus to bring the photographic properties to the desired level. At the same time, the danger of the salts crystallizing out in the coated product is avoided. In the present invention, it is also necessary to wash away the agglutinating agent.

It has been found that the washing, in general, should be conducted until the pAg, i. e., the logarithm of the reciprocal of the silver ion con centration, is between 7.0 and 9.0, and preferably between 7.5 and 8.5. Within these limits, the best balance of photographic properties, e. g., speed, contrast, fog, and stability, is achieved.

An exemplary procedure involves the use of sodium sulfate, a preferred agglutinating agent, as follows: A 10 to 20% aqueous solution of sodium sulfate is slowly added to the ripened aqueous dispersion of silver halides described above until agglutination is noticeable, followed by the rapid addition of 50 to excess of sodium sulfate solution. Agitation is continued to insure adequate opportunity for the system to reach equilibrium, the agitation is then stopped, and the mixture allowed to settle. Under these conditions the silver halide and polyvinyl alcohol are agglutinated and settle to the bottom of the vessel in the form of fine granular particles, each particle containing polyvinyl alcohol and silver halide in concentrations substantially higher than the concentrations in the original ripened aqueous dispersion of silver halide, while the supernatant liquid is clear or, at most, slightly turbid. The supernatant liquid which contains most of the soluble salts resulting from the precipitation of the silver halide and, in addition, a large portion of the water present in the dispersion before agglutination is then poured off and pure water or dilute sodium sulfate solution is added to the agglutinated mass. This is allowed to stand for a short period, e. g., 10 to 15 minutes. The supernatant liquid is again poured off and replaced with fresh Water. This type of washing is continued through several cycles, normally 4 to 5, until the pAg is between 7 and 9. The conductivity of the emulsion after such a washing procedure usually falls within the range of 200-2000 micromhos.

The agglutinate is peptized before coating to form light-sensitive layers. This is normally carried out by addition of water or an aqueous polyvinyl alcohol solution with stirring at a temperapersiorriis-i. then COSitEfiTOIItO-B. support; suchas las paper, nylon,qtransparent cellulose; ester metal, etc. Asydescribed above, it isdesirable to ripen the emulsion by heating for a time and temperature varying with the photographic properties desired. after the silver halide precipitation step and before the coagulation step. This step is important indeterminingspeed and contrast and itis: desirable to. utilize it with polyvinyl alcohol" emulsions of all but the slowest-type since. the polyvinyl alcohol does not possess' the naturalsensitizers present in gelatin and tends to give slow emulsions. The emulsion is also given a heat treatment following the pepti+ zatlon step and prior to coating. This step, normally referredtto as. digestion, has the effect of still further increasing emulsion speeds The: processes-oi this invention are illustrated by the following examples; All partsare by weight, all solutions aqueous 1 unless; otherwise stated and all preparations and handling of lightsensitive materials are carriediout in the dark and the ammonium hydroxide is 28 strength.

Example I A solution of 89.5 parts of silver nitrate. in 100 parts'of concentrated ammonium hydroxide and 800 parts of distilled water is added to a solution of 55'parts oi'ammonium bromide and 0.87 part of potassium iodide in 350 parts oi% solution of completely hydrolyzed polyvinyl acetate in 80/10 (weight ratio) water/ethanol solution, and 150 parts of waterwhile stirring in theabsence of light. The 10% aqueous alcoholic polyvinylalcohol solution had a viscosity of 41 poises at C. The emulsion is ripened by stirring at 40 C. for 60 minutes, cooled and agglutinated by the addition of anexcess of 10% sodium sulfate solution with stirring. After decanting the supernatant liquid, the agglutinated. emulsion is Washed in running Water for oneghour; To the soft a'gglutinateis added 350 partsof the same 10% solution of polyvinyl alcohol, 500 parts of water: and the whole is digestedat 65 C. for: minutes; The finishedemulsion iscoated on a suitable base; e. g'., cellulose 'nitrate film base or baryta coated paper; The emulsion layer does not wash offor reticulate when" the elements are exposed, developed, fixed, and washedin the'same manner as similarelementswith a gelatin binding agent; The emulsion layer is-much superior to the conventional gelatin: layer in resistance to cracking when bent sharply;

Example II A solution is made consisting of 6.6 parts of ammonium bromide, 03 part of potassium iodide, 0.02 part of sodium ldo'decyl sulfate in30 parts of water. Ausecond'solutionismade from. 11 parts of silver nitrate, 10 partsof ammonium hydroxide, 0.02part of sodium dodecylxsulfate and 20 parts of water. These two solutions are added simultaneously in fine streams to a stirred solution of parts of 10% aqueous solution of a completely hydrolyzed polyvinyl acetatehaving a viscosity of 12 poises at 25 C. in a 10% solution in'an 80/10 (weight ratio) water/ethanol solution, in 40 parts of water containing 0.04 part of sodium dodecyl sulfate. After ripening 15 to minutes at 40 C., the emulsion is agglutinated by the addition of an excess of acetone. The curds are collected andv washed in running; water until the mm is 8.5; To the agglutinated emulsion is added 40 parts of thesame 10% aqueous poly vinyl alcohol solution, 0.1 part of potassium bromide and the whole. stirred: at 60?- C. for. 30: to 60 minutes. The emulsion. isthencoatedion paper and. dried.. The film elementthus. pre pared is. exposed to, anegative image. and the latent image developed in; a standard p-methylaminophenol-hydroquinone developer made? by admixing the following components:

Parts p-Methylaminophenolsulfate 3.0 Sodium sulfite (anhydrous) 45.0 Hydroquinone 12.0 Sodium carbonate (anhydrous) 67.5 Potassium bromide 1.9

Water to make V 1000.0

The picture is then fixed bybathing'in a25% solution of crystalline sodium thiosulfate (hypo);v washed in running water to remove exoesssoluble salts, and dried; The finished picture is a black and white positive of good photographic quality. An outstanding property of the finished picture isits superior stability under conditions of high humidity or excessive wear by virtue of the toughness of the polyvinyl alcohol binder.

Example III A solution (A) is made from 6.6 partsof ammonium bromide, 0.3 part of potassium iodide and 20 parts of water. Another solution-(B) is made from 11.2 parts of silver nitrate, 20. parts of water, 10 parts of concentrated ammonium hydroxide and 2 parts of a 1% solution of sodium dodecyl sulfate. Solutions A and B are added simultaneously with stirring to a solution consisting of 40 parts of a 10% solution of a completely hydrolyzed polyvinyl acetate in /10 (weight ratio) water/ ethanol, 40 parts of water, and 2 parts of 1% sodium dodecyl sulfate solution. The viscosity of the 10% polyvinyl alcohol solution is 38.7 poises at 25 C. The emulsion is ripened by stirring at 42 44 C. for 15 minutes, then cooled and agglutinated by addition of'a large volume of acetone. The agglutinate is washed by immersion in running water for 30 minutes. To the soft, swollen, agglutinated particles isthen added 10 parts of the same aqueous alcoholic/polyvinyl alcohol solution, 15 parts of alcohol, 1 part of 10% potassiumbromide solution and water to give a total weight of 150 parts. at 60 C. for 30 minutes. Coating and processing are carried out as described in Example II with thesame good results. The emulsion layer and the finished picture are found to be resistant to bacterial decomposition.

ing a, viscosity of 33 poises as measured at 25 C. in a% solution in 80/10 (weight ratio) water/ethanol. and a photographic emulsionis prepared in similar fashion. When; coated. an emulsion' layerof goodaquality is formedwhich doesnot deteriorate on photographic processing even when development is carried out at .25 C.

Example V Asilver halideemulsion is precipitated. in. a

1 10% solution of completely hydrolyzedpolyvinyl acetate of viscosity 21 poises in.80/10 (weight ratio) water/ethanol at25 C. as-in Example 1; After a 60 :minute .ripeningjperiod at 40? C the unwashed emulsion is cooled and agglutination] is then accomplished by running theprecipitatedj' The emulsion is then digested bystirring emulsion as a thin stream into an equal volume of a solution of sodium sulfate. Stirring of the sodium sulfate solution is maintained during this addition which results in the formation of a large lump of agglutinated polyvinyl alcohol containing the silver halide particles. This agglutinate is prepared for washing by pressing out between rollers to give a sheet approximately V inch in thickness which is then cut up into strips about inch in Width. The process of washing out undesirable salts is then accomplished by immersing the noodle-like strips in running water for one hour. At the end of this washing period the conductivity of the emulsion has been reduced to 300 micromhos. The washed emulsion is then finished in the manner of Example I by peptization and digestion with the extra polyvinyl alcohol and water. The final emulsion, after coating on paper, shows superior resistance to abrasion as compared to gelatin emulsions during processing in the usual developing and fixing water baths. The photographic properties ofspeed, contrast, fog, and keeping qualities are sufiiciently good to render the light-sensitive paper suitable for use in projection printing.

The polyvinyl alcohols to be used in the practice of this invention are prepared by the essentially complete hydrolysis (as determined by infra-red examination) of polyvinyl esters. Suitable esters are polyvinyl acetate, propionate, chloracetate and isobutyrate. Other readily hydrolyzable esters are suitable but for reasons of economy and commercial availability polyvinyl acetate is preferred. To obtain polyvinyl alcohols of the desired high molecular weight it is, of course, necessary to employ polyvinyl esters of high molecular weight. This normally means that a polyvinyl acetate of high viscosity should be used although viscosity of the starting polyvinyl acetate has not been found to be a reliable index of the viscosity of the polyvinyl alcohol. For the purposes of this invention we have found it desirable to utilize polyvinyl acetate of a viscosity of 100-2000 centipoises measured as a molar solution in benzene at C.

The selection of the agglutinating agent is normally made on the basis of economy and efficiency of precipitation of the emulsion in a suitable form for washing. There are a large number of suitable agglutinating agents but their efficiency varies. A partial list of agents which are useful includes water-soluble electrolytes, such as sodium sulfate, aluminum sulfate, sodium chloride,'ammonium sulfate, ammonium chloride, barium chloride, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, magnesium chloride, calcium chloride, aluminum acetate, zinc chloride, zinc sulfate, and calcium acetate. These agents are also useful as coagulating or flocculating agents. a Colored materials and materials which cause fog, such as the chromium, iron, copper, cadmium, and cobalt salts and the like, are normally avoided although they can function as agglutinating agents. The preferred agents of this type are colorless, inorganic, soluble salts which have no pronounced oxidizing or reducing action and similar salts of organic acids containing less than 8 carbon atoms. Another class of agglutinating agents suitable for use in this process is the water-soluble orgame solvents which are at the same time nonsolvents for polyvinyl alcohol. Such materials include acetone, alcohol, methyl acetate, isopropanol, methanol, methyl ethyl ketone. Of these the lower aliphatic alcohols and ketones are pre-' 31 ferred. Agents which combine chemically with the polyvinyl alcohol and cause gelling or crosslinking are not comprehended by the term agglutinating agent. a

In the preparation of photographic emulsions practically any factor, such as time, temperature, rate of addition of material, rate of agitation, and the like, influences to some extent the photographic characteristics of speed, fog, and contrast of the final emulsions. For this reason wide limits in these variables are normally used depending upon the type of photographic emulsion required. Similarly the steps of ripening and. digestion are subject to a large variation. As indicated above, it has been found desirable to give substantially all polyvinyl alcohol emulsions a washing step and this washing step is an essential feature of the present process to permit the attainment of satisfactory photographic properties, e. g., speed, contrast, and keeping qualities.

The invention, moreover, is not limited to the specific light-sensitive material described in the above-detailed examples. On the contrary, various other simple and mixed silver halides may be used as the light-sensitive materials in like manner. Mixtures of silver bromides, chlorides, and/or iodides can be made by adding mixtures of soluble salts of these halides in like manner. Other useful soluble halides include potassium bromide, potassium iodide, sodium and potassium chlorides and iodides, etc. Other useful soluble silver salts include silver sulfamate, silver sulfate. silver citrate, and silver acetate.

The emulsions, before agglutination, are preferably ripened at a temperature from 40 to C. for a period of from 30 minutes to 6 hours. Silver halide solvents may be added to assist this ripening. After washing and peptization it is normally desirable to digest the emulsions for a similar period. During the digestion stage or prior thereto, various types of sensitizing agents, e. g., sulfur sensitizers, such as allylthioureas, thiocyanates, or allylthiocyanates, can be used. In addition, various types of optical sensitizing dyes which modify the spectral characteristics of the resulting emulsions can be added. Suitable sensitizing dyes are described in United States Patents 2,010,388, 2,079,376, 2,202,990, 2,202,991, 2,202,992, 2,278,461, 2,265,908, etc.

Various types of color formers or dye intermediates capable of forming quinoneimine or azomethine dyes on color-forming development can be incorporated in the emulsions. Thus, the phenols, naphthols, pyrazolones, acylacetamides, hydrindene, N-homophthalylamines, etc., particularly those of high molecular weight and those which are immobile in gelatin emulsion layers can be used. Suitable color formers are described in United States Patents 2,108,602, 2,166,181, 2,178,612, 2,179,228, 2,179,238, 2,179,239, 2,182,815, 2,184,303, 2,186,849, 2,200,924, 2,283,276, and 2,328,652.

The polyvinyl alcohol emulsions of good photographic properties as obtained herein can be used for any of the normal purposes for which gelatin silver halide emulsions are now used. That is, they may be used in the preparation of photographic paper, either for contact or projeotion printing, planographic printing plates, photographic films for various purposes, including negatives and positives, sound recording, and the like. The materials have a number of important advantages over products based on gelatin. They can be handled at higher temperatures and under unfavorable conditions such as exist in the tropics, they are much more flexible and thus can be treated roughly without fear of cracking or breaking. The materials are much more stable than the corresponding gelatin articles and are resistant to decomposition by bacteria and fungi. By the .process of this invention commercially satisfactory emulsion layers are produced which are free from the objectional color one obtains when using certain gelling agents. In addition, the photographic properties are brought under control so that emulsions of commercially feasible speed, contrast, and keeping qualities can be obtained.

As many widely diiferent embodiments of this invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not to be limited except as defined by the claims.

What is claimed is:

1. The process which comprises dissolving in water polyvinyl alcohol which contains a radical as the sole recurring radical and has a viscosity of 15 to 50 poises at 25 C. in the state of a solution of parts of said polyvinyl alcohol, 80 parts of water, and 10 parts of ethanol, precipitating light-sensitive silver halide in the resulting solution, ripening the resulting disper sion, treating the ripened dispersion with an agglutinating agent allowin the agglutinated particles of polyvinyl alcohol and silver halide to settle, removing the supernatant liquid, and washing the agglutinate until it has pAg of 7.0 to 9.0.

2. The process which comprises dissolving in water polyvinyl alcohol which contains a radical as the sole recurring radical and has a viscosity of to 50 poises at 25 C. in the state of a solution of 10 parts of said polyvinyl alcohol, 80 parts of water, and 10 parts of ethanol, precipitating light-sensitive silver halide in the resulting solution, ripening the resulting dispersion, treating the ripened dispersion with an aqueous solution of sodium sulfate while agitating the admixture, allowing the agglutinated particles of polyvinyl alcohol and silver halide to settle, removing the supernatant liquid, washing the agglutinate until it has a pAg of '7 .0 to 9.0, pepitizing and digesting the agglutinate.

3. The process which comprises dissolving in water polyvinyl alcohol which contains a radical as the sole recurring radical and has a viscosity of 15 to poises at 25 C. in the state of a solution of 10 parts of said polyvinyl alcohol, parts of water, and 10 parts of ethanol, precipitating light-sensitive silver halide in the resulting solution, ripening the resulting dispersion, treating the ripened dispersion with an agglutinating agent while agitating the admixture, allowing the agglutinated particles of polyvinyl alcohol and silver halide to settle, removing the supernatant liquid, washing the agglutinate until it is reduced to a pAg of 7.0 to 9.0, peptizing the agglutinate, adding additional polyvinyl alcohol of the same viscosity, and coating the resulting dispersion on a support so as to form a thin light-sensitive layer.

4. A silver halide emulsion obtained by the process of claim 2.

5. A silver halide emulsion obtained by the process of claim 1.

6. The process as set forth in claim 1 wherein the agglutinating agent is acetone.

ORION EDWIN SCHUPP, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,844,716 Lambert Feb. 9, 1932 2,127,573 Sheppard et al Aug. 23, 1933 2,142,311 Heidenhain Jan. 3, 1939 2,184,289 Dangelmajer Dec. 26, 1939 2,276,323 Lowe Mar. 1'7, 1942 2,311,058 Lowe Feb. 16, 1943 2,311,059 Lowe Feb. 16, 1943 

1. THE PROCESS WHICH COMPRISES DISSOLVING A WATER-POLYVINYL ALCOHOL WHICH CONTAINS A 